Flexible printed circuit boards are nowadays employed in a multiplicity of electronic devices such as mobile phones, radios, computers, printers and many more. They are constructed from layers of copper and a high-melting resistant thermoplastic: mostly polyimide, less often polyester. These FPCBs are frequently produced using adhesive tapes with particularly exacting requirements. On the one hand, for producing the FPCBs, the copper foils are bonded to the polyimide sheets; on the other hand, individual FPCBs are also bonded to one another, in which case polyimide bonds to polyimide. In addition to these applications, the FPCBs are also bonded to other substrates.
The adhesive tapes used for these bonding tasks are subject to very exacting requirements. Since very high bond performances must be attained, the adhesive tapes used are generally heat-activable tapes, which are processed at high temperatures. These adhesive tapes must not emit volatile constituents in the course of this high temperature load during the bonding of the FPCBs, which often takes place at temperatures around 200° C. In order to achieve a high level of cohesion the adhesive tapes ought to crosslink during this temperature load. High pressures during the bonding operation make it necessary for the flowability of the adhesive tapes at high temperatures to be low. This is achieved by high viscosity in the uncrosslinked adhesive tape or by very rapid crosslinking. Moreover, the adhesive tapes must also be solder bath resistant, in other words must for a short time withstand a temperature load of 288° C.
For this reason the use of pure thermoplastics is not rational, despite the fact that they melt very readily, ensure effective wetting of the bond substrates and lead to very rapid bonding within a few seconds. At high temperatures, though, they are so soft that they tend to swell out of the bondline under pressure in the course of bonding. Accordingly there is no solder bath resistance either.
For crosslinkable adhesive tapes it is usual to use epoxy resins or phenolic resins, which react with specific hardeners to form polymeric networks. In this specific case the phenolic resins cannot be used, since in the course of crosslinking they generate elimination products, which are released and, in the course of curing or, at the latest, in the solder bath, lead to blistering.
Epoxy resins are employed primarily in structural adhesive bonding and, after curing with appropriate crosslinkers, produce very brittle adhesives, which indeed achieve high bond strengths but possess virtually no flexibility.
Increasing the flexibility is vital for use in FPCBs. On the one hand the bond is to be made using an adhesive tape which ideally is wound onto a roll; on the other hand the conductor tracks in question are flexible, and must also be bent, readily apparent from the example of the conductor tracks in a laptop, where the foldable screen is connected via FPCBs to the further circuits.
Flexibilizing these epoxy resin adhesives is possible in two ways. First, there exist epoxy resins flexibilized with elastomer chains, but the flexibilization they experience is limited, owing to the very short elastomer chains. The other possibility is to achieve flexibilization through the addition of elastomers, which are added to the adhesive. This version has the drawback that the elastomers are not crosslinked chemically, meaning that the only elastomers that can be used are those which at high temperatures still retain a high viscosity.
Because the adhesive tapes are produced generally from solution it is frequently difficult to find elastomers of a sufficiently long-chain nature not to flow at high temperatures while being still of a sufficiently short-chain nature that they can be brought into solution.
Production via a hotmelt operation is possible but very difficult in the case of crosslinking systems, since it is necessary to prevent premature crosslinking during the production operation.
In many applications in the field of the production and processing of FPCBs, the adhesive tapes are removed from the release medium that normally protects the adhesive tapes, and then are positioned on the substrates to be bonded. In this case it is necessary to ensure that the adhesive tapes, which prior to this operation have frequently already been die-cut, are not deformed either during the removal of the release medium or during positioning. Since removal of the release medium requires a certain force to be expended, the modulus of elasticity of the adhesive tapes must be high enough not to experience any stretching or other deformation as a result of this force. Given that the products used must be as thin as possible, it is difficult to incorporate a carrier into the adhesive tape; instead, in general, the adhesive tape is composed only of one layer of adhesive. This adhesive, therefore, must have a sufficiently high modulus of elasticity not to be stretched.
Adhesives based on acid anhydride-modified block copolymers and epoxy resins are known from U.S. Pat. No. 5,369,167 A. A method of producing these compounded formulations is described. Moreover, curing agents are used for crosslinking the epoxy resin. There is no mention of an adhesive tape.
Similar adhesives are described in JP 57/149369 A1 as well. Again, a curing agent is needed for the epoxy resin. An adhesive tape is not disclosed.
The crosslinking of maleic anhydride-modified block copolymers with chelates is known from EP 1 311 559 A2, where an increase in the cohesion of the block copolymer mixtures is described. A way of producing heat-activable adhesive tapes which in spite of the chelate crosslinking enter at high temperatures into a crosslinking with epoxy resins is not mentioned.
Furthermore, chelate crosslinking with, for example, acid-modified acrylate adhesives is known, for example from U.S. Pat. No. 4,005,247 A or U.S. Pat. No. 3,769,254 A.
DE 10 2004 031 188 A1 discloses an adhesive tape for bonding electronic components and flexible conductor tracks, comprising an adhesive composed at least of an acid-modified or acid anhydride-modified vinylaromatic block copolymer and an epoxy resin. Metal chelates are not mentioned.
DE 10 2004 007 258 A1 discloses an adhesive for an adhesive sheet strip, comprising a mixture of                an acid-modified or acid anhydride-modified vinylaromatic block copolymer,        a metal chelate of the following formula:(R1O)nM(XR2Y)m,                    where            M is a metal from main group 2, 3, 4 or 5 or a transition metal;            R1 is an alkyl or aryl group;            n is zero or a larger integer,            X and Y are oxygen or nitrogen, and in each case may also be attached at R2 by a double bond;            R2 is an alkylene group which connects X and Y and which may be branched, or else may contain oxygen or further heteroatoms in the chain;            m is an integer, but is at least 1,                        and a tackifier resin,the adhesive sheet strips being detachable by extensive stretching in the direction of the bondline.        
The addition of epoxide-containing compounds is not described.
It is an object of the invention to provide an adhesive tape which is heat-activable, crosslinks in the heat, possesses a low viscosity in the heat, displays effective adhesion to polyimide, in the uncrosslinked state is soluble in organic solvents and has a high modulus of elasticity at room temperature.